Directly digitized still images and video requires many “bits.” Accordingly, it is common to compress images and video for storage, transmission, and other uses. Several basic methods of compression are known, and very many specific variants of these. A general method can be characterized by a three-stage process: transform, quantize, and entropy-code. Many image and video compressors share this basic architecture, with variations.
The intent of the transform stage in a video compressor is to gather the energy or information of the source picture into as compact a form as possible by taking advantage of local similarities and patterns in the picture or sequence. Compressors are designed to work well on “typical” inputs and can ignore their failure to compress “random” or “pathological” inputs. Many image compression and video compression methods, such as MPEG-2 and MPEG-4, use the discrete cosine transform (DCT) as the transform stage. Some newer image compression and video compression methods, such as MPEG-4 static texture compression, use various wavelet transforms as the transform stage.
Quantization typically discards information after the transform stage. The reconstructed decompressed image then is not an exact reproduction of the original.
Entropy coding is generally a lossless step: this step takes the information remaining after quantization and usually codes it so that it can be reproduced exactly in the decoder. Thus the design decisions about what information to discard in the transform and quantization stages is typically not affected by the following entropy-coding stage.
A limitation of DCT-based video compression/decompression (codec) techniques is that, having been developed originally for video broadcast and streaming applications, they rely on the encoding of video content in a studio environment, where high-complexity encoders can be run on computer workstations. Such computationally complex encoders allow computationally simple and relatively inexpensive decoders (players) to be installed in consumer playback devices. However, such asymmetric encode/decode technologies are not ideal for many emerging video monitoring devices and applications in which video messages are captured and encoded in real time in devices with limited computational resources.